The Dark Energy Survey: What Happens When You Can See Forever?

A new eye is now open to the cosmos. The Dark Energy Camera, which saw first light on September 12, 2012, is a spectacular new scientific facility with the grandest of goals: no less than understanding the evolution and fate of the entire universe.

For every telescope, “first light” is the moment when the optics and camera are assembled into a single instrument and turned to the night sky for the first time. But first light is just the beginning. While it often yields a spectacular photo or two, single photographs rarely lead to substantive results. Modern measurements require a subtle understanding of the equipment’s idiosyncrasies and the operators and scientists must spend a while familiarizing themselves with their instrument’s performance. After the facility has been put through its paces, real research begins. On January 9, Joshua Frieman, leader of the Dark Energy Survey (DES) collaboration, announced at the 221st meeting of the American Astronomical Society in Long Beach, California, that the team is wrapping up this getting-to-know-you phase, known as the commissioning period. They have already made interesting scientific observations, including discovering distant supernovae and clusters of galaxies.

The 570 megapixel Dark Energy Camera is hooked up to the venerable four-meter Blanco Telescope at the Cerro Tololo Inter-American Observatory, located in the Chilean Andes. Together, they will complete a study of the sky called the Dark Energy Survey, which may bring us closer to an answer to one of the deepest mysteries in cosmology: What is dark energy?

This question has been vexing astronomers since 1998, when astronomers discovered that, contrary to their expectations, the expansion of the universe wasn’t slowing down—it was speeding up! Cosmologists accounted for this surprising behavior by invoking a form of repulsive gravity first imagined by Einstein. But Einstein abandoned the idea when Hubble’s observation of the expanding universe made it seem unnecessary. Today, in the absence of a specific explanation, astronomers describe it with the generic term “dark energy.”

The Dark Energy Survey will help scientists probe the nature of dark energy. Over the course of 525 nights over five years, astronomers will survey a quarter of the southern sky to a depth of billions of light years, revealing the how the cosmic expansion rate has changed over nearly nine billions of years.

The Dark Energy Survey studies the universe in four distinct ways:

It looks for 4,000 distant supernovae. By comparing their distance (determined by simultaneously observing their brightness and their redshift, the change in their color due to the expansion of the universe, and comparing these two numbers), astronomers will get a good handle on the cosmic expansion history.

The camera will also study patterns in the spatial distribution of galaxies that are set by a phenomenon called baryonic acoustic oscillations. When the universe was smaller and hotter, the explosion of the Big Bang caused the universe to ring like a bell as the sound of the Big Bang rippled across the cosmos. About 370,000 years after the Big Bang, the universe cooled below a critical temperature, freezing these vibrations into patterns we can still see in microwave radiation and distribution of galaxies that are blazoned across the sky. This process is analogous to flash freezing the ripples on the surface of a pond. By comparing the apparent size of the ripples with their initial size, which can be calculated using information about the conditions that prevailed in the early cosmos, astronomers can provide crucial data on the shape of space itself: whether it is flat or curved and, if curved, exactly how.

The camera will also have the capacity to study the size and makeup of vast clusters of galaxies. Because the properties of dark energy help determine how and when these clusters formed, by studying their history, we can gain new insight into dark energy.

Finally, the Dark Energy Camera will see how light from distant clusters of galaxies is being bent by mass between those clusters and our telescopes. This information will tell us more about how dark energy has shaped the distribution of matter throughout the universe by studying the size and shape of clusters of galaxies over time. In total, the camera will be able to track three hundred million galaxies!

Through these four distinct strategies—each with different strengths and weaknesses—the survey will provide independent measurements of the dark energy of the universe.

The portion of the sky that the DES will study in detail is observable from Chile from September to February. Since first light, the collaboration has put their equipment through its paces. To get an early glimpse at a complete set of data, the DES collaboration will spend the rest of the 2012-2013 observation season studying a little under 5% of the region they will eventually explore. Using this strategy, they will have as good a measurement on a small portion of the sky after just a few nights of observation as they will over their entire target after five years. This will allow a relatively quick analysis of a subset of the sky and the caliber of this small study will already be world-class. The final shakedown is expected to be completed in February and in September of 2013, the survey will start in earnest, hopefully leading to new insights into the nature of dark energy.

Stay tuned. This is a very exciting time.

The center of the Milky Way galaxy lends its awesome beauty to the skyline above the telescope domes at Cerro Tololo. The Greater and Lesser Magellanic Clouds grace the upper left corner of the photo. (Photo credit: Reidar Hahn)

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Don Lincoln

Don Lincoln is a senior experimental particle physicist at Fermi National Accelerator Laboratory and an adjunct professor at the University of Notre Dame. He splits his research time between Fermilab and the CERN laboratory, just outside Geneva, Switzerland. He has coauthored more than 500 scientific papers on subjects from microscopic black holes and extra dimensions to the elusive Higgs boson. When Don isn’t doing physics research, he spends his time sharing the fantastic world of science with anyone who will listen. He has given public lectures on three continents and has authored many magazine articles, YouTube videos and columns in the online periodical Fermilab Today. His most recent book "The Large Hadron Collider: The Extraordinary Story of the Higgs Boson and Other Stuff That Will Blow Your Mind" tells the tale of the Large Hadron Collider, the physics and the technology required to make it all work, and the human stories behind the hunt for the Higgs boson.

This week, NASA announced that it will partner with the European Space Agency to send a 4,760-pound spacecraft into space to peer out over billions of galaxies in an effort to map and measure the universe. Its purpose: to investigate the mysteries of dark matter and dark energy.

Funded by the Foundational Questions Institute (FQXi) Fund, a donor-advised fund of the Silicon Valley Community Foundation.

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